This invention relates to surgical implants and in particular surgically implantable shunt devices for draining aqueous humor from an eye.
Glaucoma is a group of ocular diseases that are major causes of blindness. Most types of glaucoma are characterized by elevated pressure within the eye, the so-called intraocular pressure (IOP). Current theories suggest that this elevated IOP causes damage to the optic nerve in susceptible patients, resulting in vision impairment. Existing therapies for glaucoma attempt to decrease IOP and thus reduce optic nerve damage.
There are several approaches to reducing IOP. One type of surgical approach is to provide an alternate drainage pathway for fluid from the eye, thereby reducing IOP. Specifically, the approach is to implant an artificial shunt in the eye. The function of this shunt is to drain ocular fluid (aqueous humor) from the anterior chamber of the eye to the sub-conjunctival space overlying the eye. Such shunts are subject to two important limitations:
1) In the short term (hours to days) after surgical implantation of the shunt device, IOP can drop to very low values due to the shunt providing an essentially unrestricted egress route for aqueous humor from the eye. Until scar tissue forms at the outflow end of the shunt, this low IOP can cause sight-threatening complications and must therefore be avoided. Some existing shunts attempt to overcome this problem by inserting fluid resistors in the shunt. However, this may result in an excessively high final IOP requiring further pressure-lowering therapies.
2) Currently, there is no mechanism for selecting a long-term (months to years) level of post-operative IOP. Essentially, long-term post-operative IOP is a function of scar tissue formation and other uncontrollable variables, If IOP is not sufficiently lowered by shunt implantation, additional pressure lowering treatments are needed. No device has previously attempted to allow selection of specific target IOPs.
The use of aqueous shunts is well-established in the treatment of glaucoma, with the earliest documented procedure occurring in 1906 [Shocket, 1986]. Modern shunts are based on the design introduced by Molteno [1969; U.S. Pat. No. 4,457,757]. Essentially, Molteno""s implant consisted of a 1 mm acrylic tube attached to a round episcieral acrylic plate. The tube passed from the anterior chamber through the limbus of the eye to the plate, which was placed subconjunctivally and sutured to the sclera. As fluid drained from the anterior chamber, through the tube, and emptied onto one side of the episcleral plate, the space between the plate and the conjunctiva was maintained, forming a xe2x80x9cblebxe2x80x9d. Unlike conventional glaucoma filtration surgery, the size of the bleb was controlled by the size of the episcleral plate. By making the plate sufficiently large [Molteno et al., 1977; Heuer et al., 1992; Smith et al., 1993], the bleb filtering surface could be made sufficiently large to maintain a satisfactorily low IOP.
The most significant barrier to success of an aqueous drainage implant appears to be short-term post-operative complications [Hitchings et al., 1987; Krupin et al., 1976; White, 1992]. In particular, post-operative hypotony (sub-normal IOP) is a significant problem [Hitchings et al., 1987; White, 1992]. To address this problem, shunts have been developed that incorporate valves [Krupin et al., 1976; Krupin et al., 1988; White, 1992; Prata et al., 1995]. In principle, these valves are intended to be pressure control valves that establish a minimum IOP. However, Prata et al. [1995] tested several valve designs (Ahmed Implant, New World Medical Inc.; Krupin Eye Disk, Hood Laboratories; OptiMed Implant, OptiMed Inc.) and found that these implants were not true valves but simply flow limitation devices with no detectable opening or closing pressure. While these implants may therefore be an improvement over non-valved implants, they still do not address the essential problem of accurately controlling post-operative IOP.
The implant that is currently believed to provide some valve action is that of Ahmed (U.S. Pat. Nos. 5,071,408 and 5,411,473). The former patent states; xe2x80x9cDisclosed is a medical valve comprising a pair of plates holding in tension a membrane folded over to form a chamber with an elongated, slit-like opening along adjoining edges. The plates include interlocking members which interlock the plates together. An inlet tube in communication with the chamber extends outwardly from the plates.xe2x80x9d The membrane under tension disclosed in this patent is designed to act as a valve, in which the leaflets of the membrane are forced apart when the pressure in the eye exceeds some set pressure. However, as mentioned above, this device does not provide a true opening pressure. Also, its effective pressure range is critically dependent on the tension in the membrane, and therefore is sensitive to manufacturing issues.
Accordingly it would be advantageous to provide a shunt device that improves the accuracy and reliability of controlling and regulating the pressure within the eye when aqueous humor fluid drains from the eye through the shunt device. Further it would be advantageous to provide a similar device for use in other medical applications where drainage of fluid is required.
One aspect of the present invention is a bioinert implantable shunt device useful for controlling internal pressure. It includes a valve apparatus, a means for attaching the valve apparatus and an inlet tube. The valve apparatus includes an inlet port an outlet port, a flexible tube connected therebetween and a pressurized enclosure. The flexible tube is positioned inside the pressurized enclosure whereby fluid flow from the Inlet port to the outlet port is dependent on a differential pressure between a pressure in the flexible tube and a pressure outside the flexible tube in the pressurized enclosure. The inlet tube is connected to the inlet port of the valve apparatus. The pressurized enclosure may include a housing and a semi-permeable membrane in a wall of the housing whereby fluid can pass through the semi-permeable membrane into the pressurized enclosure thereby increasing the pressure in the pressurized enclosure. The implantable shunt device may be for use in the treatment of glaucoma.
In another aspect of the invention there is provided a method of reducing intraocular pressure including the steps of attaching a bloinert implantable shunt device to the scleral surface of an eye and inserting an inlet tube into the anterior chamber of the eye. The implantable shunt device includes a valve apparatus having an inlet port, an outlet port, a flexible tube connected therebetween and a pressurized enclosure. The flexible tube is positioned inside the pressurized enclosure whereby fluid flow from the inlet port to the outlet port is dependent on a differential pressure between a pressure in the flexible tube and a pressure outside the flexible tube in the pressurized enclosure. The inlet tube is connected to the inlet port of the valve apparatus.
Further features of the invention will be described or will become apparent in the course of the following detailed description.